This invention relates to an arrestor of an electric power transmission line and more particularly to a protective gap device for protecting a gas filled circuit breaker against steep lightning surges.
In a known gas filled circuit breaker, insulation coordination has been established by using an air gap, but in such a prior art circuit breaker the discharge voltage-time characteristic (V-t characteristic) builds up at the steep wave front of the incoming surge voltage so that the V-t characteristics vary depending upon the steepness of the wave front. For this reason it has been difficult to protect a gas filled circuit breaker having a relatively flat V-t characteristic against surge voltages having a steep wave front.
FIG. 1 shows one example of a prior art protective gap device used for a gas filled circuit breaker to obviate such defect as described above, and the protective gap device comprises a metal shell 1 filled with a gas having an excellent arc extinguish characteristic, a high voltage electrode 3, and a low voltage electrode 12 which are opposed each other in the shell 1. This protective gap device manifests a discharge characteristic having a relatively flat V-t characteristic and no polarity effect. However, in order to satisfactorily protect the gas filled circuit breaker against a steep lightning surge and to satisfactorily reclose the circuit breaker, it is necessary that the V-t characteristic should be always higher than the gentle wave front discharge starting voltage of the protective gap device and should be lower than the basic impulse level (B.I.L.) which is predetermined for the protective gap device. In the protective gap device shown in FIG. 1, although a trigger electrode 13 is provided at the center of the low voltage electrode, due to the sluggish operation of the trigger electrode and a small number of ions created by the operation of the trigger electrode and supplied between electrodes 3 and 12, the discharge between the electrodes 3 and 12 gently lags the incoming surge voltage whereby it is difficult to obtain the desired flat V-t characteristic of the gas.